1. Field of the Invention
The present invention relates to an optical transmitter, and more particularly, to an optical transmitter that transmits an optical signal to an optical receiver through free space.
2. Description of the Background Art
Conventionally, an optical space communication that transmits an optical signal through free space has been known. Since light which serves as the transmission medium in the optical space communication has directivity, it is necessary to adjust the optical axis between the optical transmitter and the optical receiver.
FIG. 21 is a view of an example of an optical space transmission apparatus having an optical axis adjusting function.
An optical transmitter 101 and an optical receiver 102 respectively have optical axis adjusting mechanisms 103a and 103b that are rotatable about a horizontal axis and a vertical axis. To enable the optical communication between the optical transmitter 101 and the optical receiver 102, the optical axis adjusting mechanisms 103a and 103b are adjusted so that the optical axes of the optical transmitter 101 and the optical receiver 102 substantially coincide with each other.
For example, when the optical receiver 102 is disposed at a point a, it is necessary for the optical axes of the optical transmitter 101 and the optical receiver 102 only to be adjusted so that the light receiving surface of the optical receiver 102 is included in a propagation plane Pa of the light emitted from the optical transmitter 101. On the other hand, when the optical receiver 102 is disposed at a point b, it is necessary to adjust the optical axes so that the light receiving surface of the optical receiver 102 is included in a propagation plane Pb smaller than the propagation plane Pa at the point a.
Thus, in the optical space transmission apparatus shown in FIG. 21, although the optical axis adjustment is easy when the distance between the optical transmitter 101 and the optical receiver 102 is comparatively long, the shorter the distance is, the more difficult the optical axis adjustment is.
Accordingly, to solve this problem, an optical space transmission apparatus as described below is known (see, for example, Japanese Examined Patent Publication No. 06-83145 (FIG. 1)).
FIG. 22 is a view of the schematic structure of a conventional optical transmission apparatus.
With reference to FIG. 22, an optical transmitter 201 and an optical receiver 202 are disposed so as to be opposed to each other. The optical transmitter 201 has: a transmission circuit unit 203; a light emitting device 204 that converts an electrical signal into an optical signal; a zoom lens 205; a drive controlling unit 206 that outputs a control signal for adjusting the zoom ratio of the zoom lens 205; and an optical axis adjusting mechanism 207a that adjusts the optical axes. The optical receiver 202 has an optical axis adjusting mechanism 207b. 
The optical axes of the optical transmitter 201 and the optical receiver 202 are adjusted by using the optical axis adjusting mechanisms 207a and 207b like the example of FIG. 21. In addition, some of the lens elements constituting the zoom lens 205 move backward and forward along the optical axis according to the control signal outputted from the drive controlling unit 206. Since the movement of the lens elements changes the spread angle of the optical signal, the area of the propagation plane of the optical signal emitted from the optical transmitter 201 changes with respect to the light receiving surface of the optical receiver 202.
Therefore, even when the distance between the optical transmitter 201 and the optical receiver 202 is comparatively short, the adjustment of the optical axes of the optical transmitter 201 and the optical receiver 202 can be facilitated by adjusting the zoom ratio of the zoom lens 205 so that the spread angle of the emitted light is increased.
However, in the optical space transmission apparatus, from the viewpoint of user safety, the optical output power is restricted. Therefore, in actuality, optical communication cannot be performed with the spread angle being increased by the zoom lens 205.
Specifically, when the optical signal outputted from the light emitting device 204 is emitted through the zoom lens 205, the light source viewed from the exit plane of the zoom lens 205 is assumed to be the point source. For example, when the light emitting device 204, which is operable to emit light of a wavelength λ of 850 nm, outputs an optical signal through the zoom lens 205, the maximum optical output power satisfied the eye safety is determined to be 0.78 mW according to the IEC60825-1 standard.
The determined optical output power is not sufficient for performing optical communication. Therefore, to efficiently use the limited optical output power, it is necessary to operate the optical axis adjusting mechanisms 207a and 207b to thereby make the optical axes of the optical transmitter 201 and the optical receiver 202 coincide with each other and then, make an adjustment to reduce the spread angle of the emitted light again by the zoom lens 205.
Further, since the substantial light receiving sensitivity of the optical receiver 202 decreases as the communication speed increases, it is desirable that the optical output power outputted from the optical transmitter be as high as possible.